The invention described herein comprises a novel planar optical structure. The structure is formed by a process for manufacturing a body from a thermoplastic plastic with a three-dimensionally structured surface, wherein molding is performed directly from a master made of glass coated with metal oxide, without deposition of further coatings on a surface of the master. The method according to the invention thus comprises fewer operational steps than corresponding conventional molding processes, which will lead to decreases in manufacturing costs. Furthermore, as a result of a smaller number of processing steps to be performed before molding on a corresponding master, risk of damage to the surface of the master, which is inevitably carried over as defects in molded bodies, is markedly reduced, which means a substantial advance in a production process.
A surface free of defects is especially important for optimization of planar waveguides, especially for applications in bioanalytics, in order to avoid scatter of guided excitation light at scatter centers and/or as a result of high surface roughness. A goal is to achieve a lowest possible surface roughness of a planar waveguide.
If in-coupling of excitation light into a waveguide takes place by use of a diffractive relief grating, then an extraordinary uniformity and reproducibility of these structures with dimensions of often only a few nanometers is necessary. In a case of manufacture of such waveguides from plastic substrates, high requirements are thus placed on corresponding molding processes.
Such periodic structures like surface relief gratings, in combination with thin metal layers deposited thereon (typically layers of gold or silver with thicknesses on the order of magnitude of about 40 nm-200 nm) on an underlying dielectric layer with a lower refractive index are also suitable for creating conditions for a surface plasmon resonance which, similar to waveguiding in an optical waveguide, is associated with formation of an evanescent field (with exponential decay in intensity in a direction of adjacent media), along propagation of the surface plasmon (instead of a guided wave). The formation of the evanescent field is explained more precisely below in an example of an optical waveguide. The term waveguiding is understood here to mean that a propagation length of a wave “guided” in a highly refractive layer shall correspond, expressed in a ray model of classical optics, at least to a distance in this layer between two total reflections at interfaces of this layer to adjacent low-refractive media or layers, opposite to one another. In a low-loss waveguide, propagation length may amount to several centimeters (or even kilometers, as in telecommunications); in a waveguide with a large-area modulated grating structure (depending on the depth of the grating) it may also measure some micrometers to a few millimeters, which is comparable with a typical propagation length of surface plasmons (typically on the order of magnitude of 100 μm). Such essentially planar optical structures, such as optical film waveguides and structures with a thin metal coating on a dielectric substrate of lower refractive index, which are described more closely below and which are suitable for generating an evanescent field, should be commonly described as “planar optical structures for generating evanescent field measuring platforms”.
The invention also relates to variable embodiments of “planar optical structures for generating evanescent-field measuring platforms”, wherein a layer (b) comprises a material from a group comprising cyclo-olefin polymers and cyclo-olefin copolymers. In particular the invention relates to a planar optical film waveguide, comprising a first essentially optically transparent waveguiding layer (a) with refractive index n1 and a second essentially optically transparent layer (b) with refractive index n2, where n1>n2, wherein the second layer (b) comprises a material from a group formed by cyclo-olefin polymers and cyclo-olefin copolymers.
The invention relates also to an analytical system with a planar optical structure according to the invention for generating an evanescent field measurement arrangement as a main component, as well as methods for manufacturing such planar optical structures and methods based on use thereof for detecting one or more analytes in one or more samples.
To achieve lower limits of detection, numerous measurement arrangements have been developed in recent years, in which detection of an analyte is based on its interaction with an evanescent field, which is associated with light guiding in an optical waveguide, wherein biochemical or biological recognition elements for specific recognition and binding of analyte molecules are immobilized on a surface of the waveguide.
When a light wave is in-coupled into an optical waveguide surrounded by optically rarer media, i.e. media of a lower refractive index, the light wave is guided by total reflection at interfaces of a waveguiding layer. In this arrangement, a fraction of electromagnetic energy penetrates into the optically rarer media. This portion is termed an evanescent or decaying field. A strength of the evanescent field depends to a very great extent on a thickness of the waveguiding layer itself and on a ratio of refractive indices of the waveguiding layer and surrounding media. In the case of thin-film waveguides, i.e. layer thicknesses that are the same as or thinner than a wavelength of light to be guided, discrete modes of guided light can be distinguished. Analyte detection methods in an evanescent field have an advantage in that interaction with an analyte is limited to a penetration depth of the evanescent field into an adjacent medium, on the order of magnitude of some hundred nanometers, and interfering signals from a depth of the medium can be largely avoided. First proposed measurement arrangements of this type were based on highly multi-modal, self-supporting single-layer waveguides, such as fibers or plates of transparent plastic or glass, with thicknesses from some hundred micrometers up to several millimeters.
Planar thin-film waveguides have been proposed in order to improve sensitivity and at the same time facilitate mass production. A planar thin-film waveguide in a simplest case comprises a three-layer system: carrier material, waveguiding layer, and superstrate (i.e. a sample to be analyzed), wherein the waveguiding layer has the highest refractive index. Additional intermediate layers can further improve action of the planar waveguide. Essential requirements placed on properties of the waveguiding layer itself and on a layer in contact therewith in a direction of the substrate or carrier material or on the substrate or the carrier material itself are in this case a maximum possible transparency at a wavelength of light to be guided, together with a minimum possible intrinsic fluorescence and a minimum possible surface roughness, in order for the light to be guided as free from interference as possible. Suitable substrate materials are therefore, for example, glass or plastics with corresponding properties, as has been widely described (e.g. in WO 95/33197 and WO 95/33198), with glass having proved more advantageous to date with regard to poverty of fluorescence (on excitation in a visible spectrum) and low surface roughness. A reason for the low surface roughness which can be achieved for glass substrates is in particular a possibility of heating these up to high temperatures so that formation of a roughness-enhancing microcolumn structure can be largely prevented.
In the case of plastic substrates, deposition of an intermediate layer between a substrate and a waveguiding layer is often necessary e.g. in order for contribution of the substrate's intrinsic fluorescence to be reduced for fluorescence measurements.
An optical waveguide with a substrate of plastic or a high organic portion and with an inorganic waveguiding layer, as well as methods for manufacture of this waveguide, are described in EP 533,074. Thermoplastically processable plastics, in particular polycarbonates, polymethylmethacrylates (PMMA) and polyesters, are preferred here.
Within this group of plastics, PMMA is known for having the best optical properties, i.e. in particular a poverty of fluorescence. A disadvantage of PMMA that has been described, however, is its low temperature stability, which does not permit continuous operating temperatures above 60° C. to 90° C., as required in some cases e.g. for nucleic acid-hybridization assays.
Less favorable physicochemical, especially optical, properties of known film waveguides comprising plastics as substrate (=essentially optically transparent layer (b)), contrasts with easier processability of these substances versus glass substrates, especially for producing a structured surface, e.g. through molding of a suitably structured master. Such molding processes for producing structured plastic surfaces generally cost less than a usual photolithographic surface structuring of glass substrates.
There is thus a need for optical waveguides, or general optical structures for generating an evanescent field measuring platform, which have similarly favorable optical properties, such as waveguides based on glass substrates, but which can be produced at lower cost.
Surprisingly it has now been found that, by using substrates of cyclo-olefin polymers (COP) or cyclo-olefin copolymers (COC), which are not mentioned in EP 533,074, it is possible to manufacture optical structures for generating evanescent-field measuring platforms and especially film waveguides, which are characterized by especially low intrinsic luminescence or fluorescence, with this being of great advantage in particular for fluorescence-based measuring methods, and which also show very low propagation losses of guided light. A new method was also surprisingly found for manufacturing film waveguides according to the invention by means of which these can be molded especially easily and in very good quality from a master.
Some favorable properties of optical components based on COP, compared with other plastics used in optics, which are listed in a product brochure of Nippon Zeon Co. Ltd., under the heading “Zeonex”, include very low water absorption, high heat resistance, low content of impurities and relatively good chemical resistance.
In U.S. Pat. No. 6,063,886, various cyclo-olefin copolymers and components manufactured therefrom by injection molding are claimed, especially for optics. However, there are no references to their use for optical waveguides with associated highly specific requirements. Also no information at all is given to indicate possible molding processes for generation of three-dimensional structures of COC.
In U.S. Pat. No. 6,120,870, an optical “disk”, based on COP, and a molding process for manufacturing this (structured) disk from a silicon master are described, wherein a resin layer between the master and a COP disk to be structured is used in each of specified variants of a molding process, with this layer being cured either photochemically, by UV light, or thermally.
In U.S. Pat. No. 5,910,287, microtiter plates (“multi-well plates”) are described in which COC or COP is used as material for a floor of wells in order to reduce intrinsic fluorescence of such a plate for fluorescence-based tests. Manufacturing processes described include reaction injection molding (RIM) and liquid injection molding (LIM).
RIM (reaction injection molding) is a low-pressure mixing and injection process in which two or more liquid components are injected into a closed mold, where a plastic body is formed during rapid polymerization. Possible problems of an RIM process that have been described are in particular blister formation during a curing process, poor mold filling and difficult demoldability of a manufactured plastic body (H. Vollmer, W. Ehrfeld, P. Hagmann, “Untersuchungen zur Herstellung von galvanisierbaren Mikrostrukturen mit extremer Strukturhöhe durch Abformung mit Kunststoff im Vakuum-Reaktionsgiessverfahren”, Report 4267 of the KfK, Karlsruhe, Germany, 1987; P. Hagmann, W. Ehrfeld, “Fabrication of Microstructures of extreme Structural Heights by Reaction Injection Molding”, International Polymer Processing IV (1989) 3, pp. 188-195). Even if these problems are largely solved, a disadvantage of the RIM process which remains is a relatively long cycle time of several minutes (T. Bouillon, “Mikromechanik—Bedeutung und Anwendung von Mikrostrukturen aus Kunststoffen, Metallen und Keramik”, study paper at the IKV, RWTH Aachen, cited in A. Rogalla, “Analyse des Spritzgiessens mikrostrukturierter Bauteile aus Thermoplasten”, IKV Berichte aus der Kunststoffverarbeitung, Vol. 76, Verlag Mainz, Wissenschaftsverlag Aachen, Germany, 1998). An LIM process (liquid injection molding) is described as even more time-consuming, with typical cycle times of 5 to 10 minutes.
In contrast to the molding processes described above, a variotherm injection process (A. Rogalla, “Analyse des Spritzgiessens mikrostrukturierter Bauteile aus Thermoplasten”, IKV Berichte aus der Kunststoffverarbeitung, Vol. 76, Verlag Mainz, Wissenschaftsverlag Aachen, Germany, 1998) is preferred for the process according to the invention in order to mold from a master of glass coated with a metal oxide, as part of a molding tool, and to manufacture a planar optical film waveguide. This purely physical process, based on liquefying at elevated temperature of plastic initially provided as pellets, enables plastic bodies to be manufactured with even very fine structures in very short cycle times (W. Michaeli, H. Greif, G. Kretzschmar, H. Kaufmann and R. Bertulait, “Technologie des Spritzgiessens”, Carl Hanser Verlag Munich Vienna 1993, p. 69).